### avg
获得某个列字段的平均值
avg($table, $column, $where)
* ##### table [string]
表名.
* ##### column [string]
列字段
* ##### where (optional) [array]
WHERE 条件.
avg($table, $join, $column, $where)
* ##### table [string]
The table name.
* ##### join [array]
Table relativity for table joining.
* ##### column [string]
The target column will be calculated.
* ##### where (optional) [array]
The WHERE clause to filter records.
Return: [number] 平均值.
~~~
$average = $database->avg("account", "age", [
"gender" => "male"
]);
echo "The average age of male user is " . $average;
~~~
